DVB-T was first adopted as a standard in 1997, and is currently rapidly expanding in Europe, Australia and Asia. DVB-T offers about 24 Mb/s data transfer capability to a fixed receiver, and about 12 Mb/s to a mobile receiver using an omnidirectional antenna. Some distinguishing technical features of DVB-T include the following: DVB-T offers a net bit rate (R) per frequency channel in the range of about 4.98 to 31.67 Mbit/s and operates with a channel separation of 8 MHz in the UHF range of 470-862 MHz(in the VHF 174-216 MHz range the channel separation is 7 MHz). Single frequency networks can be used. DVB-T uses a Coded Orthogonal Frequency Division Multiplex (COFDM) multi-carrier technique with QAM, 16QAM or 64QAM carrier modulation. The number of sub-carriers can be between 1705 (2 k) to 6817 (8 k). Inner forward error correction coding (FEC) uses convolutional coding with rates ½, ⅔, ¾, ⅚ or ⅞, while an outer coding scheme uses Reed-Solomon (204,188,t-8) coding. Outer bit-interleaving uses convolutional interleaving of depth 0.6-3.5 msec.
While DVB-T was developed for MPEG-2 Transport stream distribution, it is capable of carrying other types of (non-video) data. For example, DVB-T can provide a broadband, mobile wireless data transport for video, audio, data and Internet Protocol (IP) data. DVB-T is scalable, with cells sizes ranging from, for example, 100 km down to picocells (e.g. tens to hundreds of meters). The capacity is very large, e.g, 54 channels can be supported, each running at 5-32 Mbit/s. One TS-packet is 188 (204) bytes long.
Due to the large number of sub-carriers the symbol time can be made very long. For example, for the 8 k sub-carrier case the symbol time is on the order of 1 millisecond. A guard interval is inserted before each symbol.
Thus, it can be realized that while well suited for providing digital video streams, DVB-T can be used to provide high speed data streams for other types of applications, such as interactive services, Internet access, gaming and e-commerce services. As can be appreciated, for interactive and other services to be provided a return link or channel is required from the user back to some server or other controller. One example of such as a system is known as MediaScreen™ that was shown by the assignee of this patent application. This device provides a LCD display screen for displaying information received from a DVB-T downlink, and includes a GSM function having a transmitter to provide the return link or channel.
In WO 01/39576, “Charging in Telecommunication System Offering Broadcast Services”, published 7 Jun. 2001, Risto Mäkipää and Jorma Havia (Alma Media Oyj) describe a system having a terminal and a broadcast network offering broadcast services. The terminal selects the information to be broadcast by means of a reverse connection made through, for example, a third generation mobile system, embodied as a Universal Mobile Telecommunications System (UMTS) network.
A problem that may be created by the transmission of the DVB-T signal is interference into the UMTS receive band (beginning at about 826 MHz). This problem was recognized and discussed by C. Hamacher “Spectral Coexistence of DVB-T and UMTS in a Hybrid Radio System”, ComNets, and the use of a guard band (GB) is discussed. FIG. 1 of this patent application is based on FIG. 1 of Hamacher, and shows an adjacent channel interference (ACI) scenario, with transmitter masks defined by the relevant DVB-T and UMTS standards, and the receiver filters assumed to be ideal. In Section VI (Conclusions and Future Work) the author states that comparable studies with DVB as a victim system would be performed.
In an article entitled “Evaluation of Packet-by-Packet Downlink Radio Resource Management Schemes”, in VTC′01, Rhodes, Greece, Jun. 6-9, 2001, and in an article entitled “Dynamic Single Frequency Networks”, IEEE Journal on Selected Areas in Communications, Vol. 19, No. 10, Oct. 2001, pgs. 1905-1914, Magnus Eriksson discusses asymmetric Internet access using a DVB-T downlink with a cellular system, i.e., GSM, as the narrowband uplink. These articles discuss the use of dynamic radio resource management (RRM) techniques, such as dynamic channel allocation (DCA), link adaptation and traffic adaptive handover to improve spectral efficiency.
The inventors have realized that a potential exists for a problem in the DVB-T receiver when an associated return channel cellular system (e.g., GSM) transmitter is operational, especially in the case where there is only a small physical separation between the two antennas (i.e., the two antennas are operating in the near field, and antenna radiation pattern filtering cannot be employed in the receiver filtering arrangement.) Furthermore, this problem is not limited to the use of GSM for the return channel, but can occur as well should a GSM voice call or a data call be made when DVB-T reception is ongoing. For example, the user might perform a digital packet access via a GSM/EDGE network to an e-mail server or a similar packet protocol system. Furthermore, a WAP communication can be made during DVB-T reception to view a schedule of television programming that is available from a WAP/WEB server.
The problem arises because the lower end of the GSM transmission band begins at 880 MHz, while the upper end of the received DVB-T frequency band ends at 862 MHz. Thus, transmitted energy from the GSM band can leak into the DVB-T receiver, resulting in errors in the received data. This is shown graphically in FIG. 2. The point labeled as A designates the GSM 900 MHz −23 dBm receiver blocking level used for an in-band blocking measurement. The spurious in-band blocking specification to one tone is −23 dBm at 3 MHz, and −31 dBm at 6 MHz. If one assumes that the GSM900 average transmitted power is +33 dBm, with reference to FIG. 3, and assumes a reasonable 6 dB of antenna isolation from the GSM antenna 20 to the DVB-T antenna 12 (an exact figure is difficult to discern, as the antennas are assumed to be in the near field, and antenna pattern filtering is not usable), then the power seen at the input to the DVB-T receiver 14 is +27 dBm, which is more than 30 dB greater in spurious signal level than in the GSM receiver 22. In FIG. 2 the delta (Δ) indicates the more strenuous (50 DB) difference in the required DVB-T blocking requirement. The DVB-T receiver sees significant GSM transmitter noise in the 8 k sub-carrier band at the upper (862 MHz) end of the DVB-T spectrum. This is an undesirable situation, as errors can be experienced in the DVB-T reception when the GSM transmitter is active.
While at first glance it may seem that one could simply implement a highly linear DVB-T receiver, in practice this is difficult to achieve in a cost effective and a low power consumption manner, both of which are important considerations when building portable, battery powered consumer devices. If the GSM transmission from the lowest GSM transmit channel were to be adequately filtered from the DVB-T receiver when operating at the highest channel, a very steep filter would be required. The steepness of the required filter implies that the insertion loss at the passband of the DVB-T receiver is increased, and thus the sensitivity of the receiver would be reduced.
It should be noted that while the foregoing discussion has concentrated on specific DVB-T frequencies and the European GSM system, the same problems can arise in other locations where Digital Television has been specified for use. For example, in the United States of America digital television is referred to as ATSC (Advanced Television Systems Committee), and the FCC has allocated the frequency bands of 764-776 MHz and 794-806 MHZ for Digital Television (DTV) broadcasts. One U.S. cellular transmission band (already occupied) has been established from 824-849 MHz. As can be noted, the upper boundary of the DTV band (806 MHz) is separated from the lower end of the cellular transmit band by only 18 MHz, about the same separation that is seen in the DVB-T/GSM embodiment described above.